It is known in the art that "shearography" (also known as, image-shearing, speckle-pattern interferometry or speckle-shearing interferometry) consists of interfering an image of an object illuminated by laser light with itself after a small amount of lateral displacement (or shear) has been introduced thereto. Shearography is a technique for measuring phase changes between two optical fields having random amplitude and phase distributions obtained when a surface is illuminated with laser light and imaged by a camera through a shearing mechanism, as described in the article: J. A. Leendertz et al, "An Image Shearing Speckle Pattern Interferometer for Measuring Bending Moments", J. Phys. E. Sci. Inst., Vol.6 (1973), pp 1107-1110, and the article: Y. Y. Hung et al, "Measurement of Slopes of Structural Deflections by Speckle Shearing Interferometry", Exp. Mech., Vol.14 (1974), pp 281-285.
Typically, the image is sheared, as is known, so that the speckle pattern from one point on the surface of the object can be made to interfere with the speckle pattern from a neighboring point. The resultant interference pattern is recorded and used as a reference. The pattern is random and depends on the characteristics of the surface of the object under study.
A second interference pattern is obtained when the object is stressed or deformed by temperature, pressure, or other means. The first (reference) interference pattern is made to interfere with the second (stressed) interference pattern by a known means called correlation interferometry. Both interference patterns comprise random speckles, but at regions where the phase of the interfering fields in the second pattern differ by an integral number of wavelengths from the first pattern, the speckles in the second pattern will correlate with the speckles of the first. Similarly, between these regions, the speckles are not correlated (decorrelated).
Interfering the two interference patterns produces, in general, cyclical alternations of correlation and decorrelation across the image of the object. When these alternations are made visible, the resultant image is a "fringe pattern" (i.e., regular alternation of lightness and darkness) that may be used to measure the deformation of the object.
It is also known in the art to use double exposure photography combined with Fourier transform plane filtering in order to make the resultant fringe pattern visible to the observer. Further, it is known that video frame storage and real time video subtraction provides an advantageous method of generating speckle correlation fringes. Anywhere two speckle patterns are correlated, the subtracted image comprises black pixels. Conversely, where the patterns are decorrelated, the subtracted image comprises a high percentage of white pixels. This is known in the art and is described in U.S. Pat. No. 3,816,649 to Butters et al.
A number of optical configurations have been proposed for providing the sheared images for shearographic analysis. The apparatus described the aforementioned article by Leendertz et al is essentially a Michelson interferometer, i.e., a partially reflecting beamsplitter and two mirrors substantially perpendicular to each other with one mirror slightly tilted relative to the other. The output of the beamsplitter provides two images having the same orientation, yet shifted laterally with respect to one another. Speckle pattern interferometry is then used to measure phase changes between the two images resulting from deformation of the surface.
Another apparatus comprises a lens with two apertures and a pair of glass blocks used to divert the rays passing through the lens apertures, as described in the aforementioned article by Hung et al. A third configuration includes a birefringent material (such as a calcite crystal) to provide the shearing effect, followed by a single lens which is followed by a polarizer, as described in U.S. Pat. No. 4,887,899 to Y. Hung. A fourth configuration includes a beamsplitter in combination with a single mirror as described in U.S. Pat. No. 5,094,528 to Tyson II, et al.
Each of the aforementioned configurations have an imaging lens (such as a TV camera lens), that forms the image on an image sensing device, located at the output of the shearing element (i.e., the optical element(s) used to provide the sheared image).
This arrangement has an inherent disadvantage in that the shearing element must be sized to allow viewing of the region on the object to be imaged. Therefore, the size of the shearing element must be significantly larger than the entrance pupil of the TV camera lens to image a large area of the object. Consequently, the size of the shearing element determines the angular field of view of the imaging lens, thereby making wide-angle viewing of the object, size prohibitive.